Technical Field
The present disclosure is directed to power converters and, more particularly, switch-mode power converters having a regulated output voltage or output current.
Description of the Related Art
Power converters are very common devices for translating electrical energy from one form to another. The conversion of energy from one form to another can take place in a variety of different ways, such as ac-dc, dc-dc, ac-ac and dc-ac power converters. Inherent to all power converters is control of the converter, generally through a control circuit.
The block diagram of FIG. 1 shows a general structure of a power converter 20, located between a power source 22 and a load 24. The power converter 20 includes a power stage 26, sometimes termed “power circuit,” and a control unit 28, which is connected to the power stage 26 and controls the power stage operation based in part on measurements of the output voltage or current produced at the load 24. The power converter 20 receives power from the source 22 and converts the electrical energy to a different form to apply to the load 24.
The power stage 26 can be implemented by many power stages known in the art and generally includes a combination of power semiconductor devices and passive components, mainly transformers, inductors, and capacitors. In some converters, the power stage involves multiple conversion stages using the same or different topologies connected either in a cascade or in parallel.
The control unit 28 receives measured electrical quantities, which are representative of the output voltage Vout or output current Iout, in the power stage 26 through measurement systems 29. Based on the measured quantities received, the control unit 28 outputs control signals that enable the power stage 26 to modulate and control the energy flow, effectively regulating the electrical quantities.
Modern power conversion is usually based on switch-mode technology, where one or more switches are used to close or open branches in the power circuit 26 at a switching frequency, fsw, to control energy flow. The switches are generally power semiconductor switches so the control quantities output by the control unit are two-level pulsed signals that determine the open and closed state of the controllable power switches. The power switches may be any suitable semiconductor device for handling high-power switching operations, such as bipolar junction transistors (BJTs), field effect transistors (FETs), and insulated gate bipolar transistors (IGBTs). The switches can also include diodes.
For dc-dc converters, the control unit 28 functions to keep the dc output voltage Vout or the dc output current Iout constant, even under changing operating conditions. Operating conditions may change as a result of changes in the dc input voltage Vin or changes in the output power demanded by the load 24. The control unit 28 ensures that the output quantity to be regulated (either Vout or Iout) is as close as possible to a preset constant value, also referred to as the setpoint.
FIG. 2 illustrates a control unit 28 having a common closed-loop, negative-feedback control design. The control unit 28 of FIG. 2 includes four major blocks: a sensing circuit 30, a signal conditioning circuit 32, a modulator 34 and a driver 36.
The sensing circuit 30 measures the quantity to be regulated, generally either the output voltage Vout or the output current Iout, and produces a measured value signal X representative of the quantity to be regulated, as well as other electrical quantities in the power circuit that are used to perform the control action. The measured value signals X are then transmitted to the signal conditioning circuit 32, while some additional electrical quantities are transmitted to the modulator 34.
The signal conditioning circuit 32 processes the measured value signal X coming from the sensing circuit 30. In particular, the signal conditioning circuit receives the measured value signal X, and generates either a control voltage Vc or a control current Ic based on the measured value signal X. As in most closed-loop, negative-feedback control systems, the signal conditioning circuit 32 includes a frequency-compensated error amplifier 38. The frequency-compensated error amplifier 38 is generally an operational amplifier surrounded by a passive network.
The modulator 34 combines the control signal (either Vc or Ic) with additional signals produced by the sensing circuit 30 to modulate a quantity Ψ, which the power stage 26 ultimately uses for control of the energy flowing through the power stage. Ultimately, the modulator 34 generates a train of low-power two-level pulsed signals qj(t) which are received by the driver 36.
The driver 36 is generally a power amplifier and a level shifter which receives the low-power inputs qj(t) and produces the higher power signals Qj(t). The signals Qj(t) have an amplitude and a power level suitable to drive the power switches of the power stage 26.
When the operating conditions of the converter change, any deviation in the regulated quantity Vout or Iout from the setpoint produces a change in the control signal Vc or Ic. This change in the control signal results in a change in the quantity Ψ, which balances the input-to-output energy flow. This balance ensures that the regulated quantity Vout or Iout remains as close as possible to the setpoint.
In order to achieve the control regulation of the output quantity Vout or Iout, the control system is designed to ensure a stable control loop, good regulation, and good dynamic performance. A stable control loop can be met by recovering the regulated quantity Vout or Iout to a steady-state value after disturbances of the operating conditions of the converter have finished. Good regulation is met when both the constant value of the regulated quantity Vout or Iout before the disturbance and the new constant value of the regulated quantity Vout or Iout following the disturbance are as close to the setpoint as possible. Finally, good dynamic performance is achieved when the regulated quantity Vout or Iout does not excessively deviate from the setpoint and the transient itself fades away in a short time.
These control objectives may be expressed in terms of characteristic quantities of the transfer function of the control loop, such as bandwidth, phase margin and dc gain. The objectives can be achieved by acting on the frequency response of the error amplifier network 38 in the signal conditioning circuit 32, such as modifying its gain and appropriately placing the poles and zeroes of its transfer function. This may be achieved by selection of the value of resistors and capacitors that make up the passive network attached to the amplifier.
Many different methods exist for controlling the regulation of the output quantity Vout or Iout. One group of methods are based on pulse-width modulation (PWM), and include methods such as “duty cycle control,” “peak current-mode control,” and “average current-mode control.” The duty cycle control method sets the quantity Ψ as the ratio between the time during which a power switch is closed, TON, to the switching period Tsw. The peak current-mode control method sets the quantity Ψ to the peak current flowing through the energy storage magnetic device. The average current-mode control method is similar to the peak current-mode method, but sets the quantity Ψ to the average current flowing through the energy storage magnetic device.
In addition to PWM control methods, there are also pulse frequency modulation (PFM) methods, where the switching frequency fsw is variable. These include the “direct frequency control” method, where Ψ is the switching frequency of the converter; and the “time-shift control” method, where Ψ is the amount of time from a zero-crossing of the current in the energy storage magnetic device to the next change of state of the power switches.
Another important characteristic of the power circuit 26 that impacts how the control circuit is implemented, in particular the way the control signal (Vc or Ic) is passed on to the modulator, is whether the converter is isolated or non-isolated. This “isolation” refers to the existence of an electrical barrier between the input and output of the converter.
FIG. 3A shows a non-isolated converter 40, which has a common ground terminal for both the input and the output. The electrical connection between the input and output make them simple and cost efficient, but limits their usage to certain applications, such as Point Of Load (POL) converters.
Non-isolated converters do not need any special electrical provision to provide the control signal to the modulator. If the circuits are properly combined, the output of the signal conditioning circuit can be connected directly to the modulator input.
However, many safety agency bodies or customers require a separation from the applied input voltage and the output voltage, which is often user accessible. FIG. 3B shows such an isolation barrier 42 for a dc-dc converter 44. The isolation barrier 42 is a high frequency transformer, which removes the direct electrical connection from the input to the output.
With isolated converters, the power is switched on the input side (commonly referred to as the primary side), but under control from the output side (commonly referred to as the secondary side) in order to provide proper regulation. This requirement introduces an additional problem, namely that signals from the secondary side are transmitted to the primary side. The requirement for primary side switching to be controlled by secondary side characteristics requires a second connection crossing the isolation barrier in order to feed the control signal (Vc or Ic) back to the primary side. Although this path involves only information, rather than power, it should still be isolated.
FIG. 4 shows a common inexpensive solution to this problem. In this arrangement a three-pin adjustable shunt regulator 46 is used as secondary reference/error amplifier that drives an optocoupler 48 to regulate the output voltage and transfer the control signal to the primary side.
With this circuit arrangement output voltage changes Δ Vout are represented by corresponding changes ΔIΦ in the current IΦ flowing through the photodiode. The current IΦ determines a proportional change ΔIc in the current Ic drawn by the phototransistor. This current drives the modulator directly, or is first converted into a voltage before being fed into the modulator.
Another solution to avoiding isolation problems is to eliminate the feedback. An example of one prior art approach to a “no overall feedback” converter is shown in FIG. 5. This converter 50 uses the same low-voltage, primary-referenced auxiliary winding that supplies power to the control circuit, but in this case a non-isolated feedback loop is used to force the control IC to regulate its own supply voltage. The theory is that if the diode voltage drops are matched, and the transformer windings well coupled, the isolated output voltage will track this regulated primary-referenced auxiliary voltage. However, the performance offered by this approach, commonly referred to as Primary Sensing Regulation (PSR), in terms of regulation and accuracy is worse than the circuit of FIG. 4.